This invention relates to pyrometer apparatus.
Pyrometers generally comprise a heat-resistant lens that focuses radiation emitted by the object under observation onto a radiation detector. In some apparatus the radiation detector is mounted remotely, radiation being focused by the lens onto one end of a fibre-optic cable that is used to transmit radiation to the detector. One problem with such pyrometers, in certain environments, is that the forward surface of the lens can become coated with contaminants, such as combustion products, thereby altering the amount of radiation passed to the detector. Various attempts have been made to reduce contamination of the lens and to compensate for any contamination that does occur.
One proposal is to provide compensation by viewing the front surface of the lens so as to monitor the amount of contamination. So that this can be done while maintaining observation of the object, an image of the object is focused using a pin-hole aperture. It will be appreciated that the nature of a pin-hole aperture is such that the image is focused in all planes behind the aperture and, more particularly, on the front surface of the lens. In this way, radiation from the object can be monitored at the same time as the front surface of the lens. The problem with such an arrangement, however, is that the pin-hole aperture reduces the amount of radiation passed to the detector. If the size of the aperture is increased so as to pass more radiation, the resolution of the apparatus is correspondingly reduced.